The invention relates to a semiconductor device for radio frequencies of more than 10 GHz and to a method for producing the device.
In one embodiment, the semiconductor device has a semiconductor chip which, on its active top side, has a radio-frequency region and a low-frequency region and/or a region which is supplied with DC voltage. In this case, the low-frequency region and/or the region which is supplied with DC voltage of the semiconductor chip may be embedded without problems in a plastic housing composition having a relative permittivity of 3 to 5. In the case of electronic components to which radio frequencies are applied, such materials of a plastic housing composition in the direct surroundings of the conductor tracks influence the electrical properties of the radio-frequency device and considerably restrict the processability and integrity of radio-frequency signals.
In this case, the dielectric constant ε, where ε=εr·ε0, ε0 being the absolute permittivity in free space and εr being the relative permittivity, influences the signal propagation speed which is proportional to ε−1/2. Therefore, low values of the relative permittivity εr are strived for for radio-frequency semiconductor components in order to achieve fast propagation speeds and thus short delay times. In addition, the dielectric constant influences the impedances of the conductor tracks since the impedances increase approximately in proportion to ε. Another physical factor which is dependent on the surrounding material is the loss factor tanδ. The loss factor tanδ determines the dispersion or distortion of a signal. A high loss factor causes the signal to disperse, with the result that a square-wave pulse does not retain its shape. Therefore, there is also the need for a low loss factor for radio-frequency devices in this case since, for example, the square-wave pulses then retain their shape in virtually undistorted fashion during the propagation time over a prescribed distance.
The current plastic housing compositions including filled plastics such as duroplasts or thermoplasts have a typical value of the dielectric constant or the relative permittivity εr of approximately 3 to 5 and a loss factor tanδ of approximately 0.01. In this case, these values depend both on the temperature and on the frequency, the above values relating to radio frequencies of approximately 1 GHz and the interaction with the surrounding material and the effect on the electrical properties becoming ever more critical as the frequency increases. On the other hand, it is not possible to dispense with a plastic material for protecting the surface of the radio-frequency semiconductor chips and the integrated circuits on these semiconductor chips in order to ensure the reliability and lifetime of such semiconductor devices. The abovementioned material properties as regards the dielectric constant εr and the loss factor tanδ limit the applicability of conventional housing technologies to certain radio-frequency applications, the radio-frequency properties of the semiconductor components being greatly restricted and deteriorating as of a certain cut-off frequency.
These cut-off frequencies have hitherto been a few GHz, with the result that there is a need to push out these cut-off frequencies further into the range of several 10 GHz to a few 100 GHz. In order to push out these limits, use is preferably made of cavity housings in which the internal electrical connecting elements such as wire connections are now no longer encapsulated by plastic with the disadvantage that such exposed wire connections greatly reduce the reliability of the semiconductor components. Other solution approaches operate using flip-chip contacts in order to avoid the wires drifting and to shorten the length of the connecting elements. However, even these run into cut-off frequencies, depending on the size of the flip-chip contacts, if the signals are radio-frequency signals of more than 10 GHz.
The document U.S. Pat. No. 6,446,316 B1 discloses the encapsulation of SAW components, “surface acoustic wave” filters. In this case, during encapsulation, a cap is placed onto the semiconductor substrate in order to protect the radio-frequency components from contact with the plastic housing composition. The cap has the form of a cover and has cutouts in which the radio-frequency components of the SAW filter are arranged. In order to produce such a cover which has been patterned with cutouts, a layer which can be patterned by means of photolithography is applied to the cover and is then patterned by means of exposure and development and applied to the substrate including semiconductor material. A cover which has been patterned with cutouts in this manner requires a manufacturing method which is associated with high costs especially since the alignment of the cover on the substrate also gives rise to problems.
For these and other reasons, there is a need for the present invention.